Electrode wire



Patented June 29, 1943 UNITED STATES EATENT OFFICE ELECTRODE WIRE ofDelaware No Drawing. Application May 1941, Serial No. 392,947

2 Claims.

My invention relates to electrode wires of nickel base alloys andparticularly of such alloys in which nickel is present in amountsgreater than 90% of the alloy, which have characteristics such that thealloys make good electrodes for electron discharge devices and the like,and which are ductile enough to be drawn into fine wire.

Nickel is used extensively for the electrodes of radio tubes. Toincrease the strength of the nickel for fine wire electrodes variousmetals have been alloyed with the nickel. The addition of such metals assilicon, magnesium, manganese, chromium and even the more refractorymetals, such as zirconium, titanium and tungsten, added separately aswell as in various combinations will in most cases increase the tensilestrength of the nickel so that it may be drawn into wire. However, thiswire, when used as electrodes in some radio tubes, fails at elevatedtemperature even though the tested cold tensile strength may becomparatively high. Fine grid Wires, for example, may sag during hightemperature exhaust and filaments operated at cathode temperatures mayburn out after a short life.

An object of my invention is a fine wire electrode for radio tubes andthe like composed of an improved alloy.

Another object of my invention is an electrode wire of an improved alloyfor wire that may be drawn to very small sizes and which has high hotstrength.

A more specific object of my invention is an electrode wire of a purenickel alloy for filamentary cathodes.

Another specific object of my invention is an electrode wire of a purenickel alloy for fine Wire grids.

The characteristic features of my invention are defined in the appendedclaims and a preferred embodiment thereof is described in the followingspecification.

I have produced, according to my invention, a nickel base alloy whichwhen drawn into fine wire has unusually great hot strength and is welladapted for use as a base for oxide coated cathodes and for fine wiregrids. The nickel base metal of my improved alloy contains a smallquantity of tungsten and carbon, Which I believe are present as tungstencarbide, either as WC or W2C. By addition of these two elements to thismetal I am able to produce a wire that has a cold tensile strength ofmore than 100 grams per milligram per 200 millimeters compared to thetensile strength of only 45 grams per milligram per 200 millimeters forone commercial nickel widely used for electrodes and containing 1%aluminum and .4% silicon, and has a hot or burn-out strength, especiallyin fine wire sizes, much greater than the same size wire made of thiscommercial nickel. One specific alloy which embodies my invention andwhich I have found to have high hot strength, although operated atcathode emission temperatures for many hours, consists of, by weight,

Per cent Carbon .15 Tungsten 2.0 Nickel remainder As more fullyhereinafter pointed out, the specified percentages of carbon andtungsten may be varied within reasonable limits Without materiallychanging the chemical and mechanical properties of the alloy and withoutdeparting from the scope of my invention. My alloy is so ductile ndtough that it can be drawn, without breakage, into long lengths of wireas small as eight ten-thousandths of an inch in diameter. It is mybelief that carbides are formed by the tungsten and carbon and that theproportions above mentioned are optimum for producing a smooth and toughalloy texture.

While the addition to the nickel base of 2% tungsten with about .15%carbon appears to produce an optimum quantity of carbide for highstrength, the proportion of these constituents may be varied withinlimits without adversely affecting the properties of the alloy. Theaddition of more than 2 or 3% tungsten does not materially improve thehot strength or emission properties of the alloy while tungsten inquantities less than about .66% causes a drop in the strength of thewire. The total carbon, combined and free, should not be present in thewire in quantities less than .05% in order that suflicient of thetungsten may be converted to tungsten carbide, and the carbon in thefinished wire should not exceed .5% because of the difiiculty of drawingthe wire. The usual methods of chemical analysis do not reveal theproportion of free and combined carbon, although the total may bedetermined.

In preparing my nickel alloy, cubes of commercial electrolytic nickel,which by the usual methods of commercial analysis is substantially pure,free from sulphur, and has only traces of such metals as iron, cobalt,magnesium, silicon and copper, are melted in a magnesia crucible at 1560to 1600 C. Then pieces of a pressed rod of mixed carbon and nickelpowder are added to the melt until the melt becomes completelydeoxidized and is quiescent. Additional carbon is then added to carrythe nickel through the swaging hammers and through the rolls and wiredies and leave the wire with a final carbon content of, preferably,about .15%. Experience has shown that, after deoxidation, about 20%, byweight, of additional carbon should be dropped into the melt from theabove mentioned rods to produce a final wire with .15% carbon, theamount of carbon being dependent somewhat on the rolling, swaging anddrawing technique. Then the tungsten, which may for convenience be inthe form of broken pieces or ends of a pressed unsintered bar of puretungsten powder, is added to the melt, which is held at about 1600* C.Usually in about ten minutes the tungsten is completely alloyed with oris completely dissolved in the nickel. The melt is then promptly pouredinto molds to form the usual slugs, which may be swaged and drawn intofine wires by the conventional methods for working nickel. By thismethod a true alloy is formed and not merely a sintered mixture andconsistently good results have been obtained in drawing thousands ofmeters of wire only eight tenthousandths of an inch in diameter.

. The hot strength of my improved alloy may be tested by operating afilament of the alloy in a radio tube at a filament voltage twice therated voltage, and noting the time required for the filament to burnout. The time required for burn-out of my alloy was more than twice thetime required for burn-out of a similar nickel alloy cathode containing.2% magnesium, .2% silicon and .15% carbon, indicating that my alloy hasa lower vapor pressure and lower rate of evaporation.

My nickel-tungsten-carbon alloy is particularly adapted for use asfilaments in electron discharge devices when aluminum is added asexplained in my copending application, Serial No. 458,867 filedSeptember 18, 1942, which is a continuation-in-part of this application.

My improved alloy has high hot and cold strength, and may be drawn tovery small wire sizes.

I claim:

1. An electrode wire for electron discharge devices, said electrode wirecomprising an alloy containing about .15 percent carbon, about 2 percenttungsten and the balance nickel, said wire being characterized by anincreased hot strength and an increased resistance to burn-out byelectric heating in vacuum as compared to a wire of nickel withouttungsten.

2. An electrode for radio tubes and the like comprising a drawn wire ofan alloy containing, by weight, .05 to .5 percent carbon, .66 to 3percent tungsten and the balance substantially of nickel, said wirehaving a higher tensile strength in vacuum than said wire of nickelwithout the tungsten.

EMIL GIDEON WIDELL.

